Method and apparatus for shaping a normally rigid plastic pipe

ABSTRACT

A normally rigid plastic pipe is shaped by softening a selected zone of the pipe, inserting into the softened zone a resiliently deformable mandrel and pressing axially on opposite sides of the mandrel. This expands the mandrel radially into frictional engagement with the softened pipe zone. Contemporaneously the mandrel is shortened axially, thereby correspondingly expanding radially and shortening axially the softened portion of the pipe while proportionately thickening its side walls.

This is a division of application Ser. No. 363,081, filed May 23, 1973,now U.S. Pat. No. 3,923,952.

This invention relates to a method and apparatus for shaping a normallyrigid plastic pipe or tube. It pertains particularly to a method offorming a bell or socket on the end of a synthetic thermoplastic pipesuch as is employed in the transmission of fluids, as an electricalconduit and the like.

Pipes made of heat softenable synthetic plastics are well known andwidely used for many purposes. They customarily are supplied in standardlengths and diameters. To enable joining successive lengths end to endit is conventional practice to heat-soften one end of each length andthen form it with a mandrel to an enlarged diameter. This forms anintegral bell or socket dimensioned to receive the unaltered end of anadjacent pipe length in telescoping relationship. The telescoped endsthen are sealed with cement or by means of O-rings to form the finishedjoint.

Where an integral bell is formed in the manner described, thereobviously is a thinning of the wall in the formed zone resulting fromstretching it to an increased diameter. This thinning is inconsequentialwhere the pipe is to be used at ambient pressures, as is the case whereit is used for electrical duct work, or as drain pipe.

However, if the pipe is intended for use in situations in which it issubjected to pressure, as in pressurized water systems, thinning of thepipe at the bell is unacceptable because the pipe strength is reducedcorrespondingly. In such situations it is required that the wallthickness of the bell be at least as great as the wall thickness of thebalance of the pipe.

Various expedients heretofore have been employed to provide pipe havinga bell of adequate wall thickness.

In one, a molded bell fitting of the desired wall thickness is lappedover the end of a length of pipe and cemented thereto. This provides thedesired bell wall thickness and the exterior detailing of the bell issharp and workmanlike. However, the cemented-on bell is less reliablethan an integral bell because cementing the molded bell onto the pipe isdifficult, particularly with the larger sizes and the workmanshipinvolved in making the joint may be unreliable. Because of the addedlabor, the cost is more. Because of the necessity of providing amultiplicity of expensive injection molds, one for each bell size, thenumber of sizes commercially available is likely to be restricted to themost popular sizes only.

A second procedure for manufacturing plastic pipe with bells of adequatewall thickness is set forth in U.S. Pat. No. 3,264,383.

In accordance with this method, plastic pipe is extruded with spacedsegments of increased wall thickness. The pipe is cut at thethick-walled areas which then from the pipe ends. These ends then areenlarged over a mandrel to the desired bell shape. The increasedthickness of the original pipe end thus compensates for the thinningoccurring when it is forced over a mandrel. This method obviouslyrequires complex and expensive machinery to manufacture the extrudedpipe with spaced segments of increased thickness.

Still another procedure for achieving the desired result is to cement anexternal collar of plastic over the ends of the pipe which are to bemandrel-belled. This procedure is workable, but requires careful controlof collar dimensions and placement with resultant increased costs.

It accordingly is the object of the present invention to provide methodand apparatus for shaping a normally rigid plastic pipe or tube,specifically the end of a thermoplastic pipe, which overcomes theforegoing problems and provides an integral bell having a wall thicknesswhich is as great as or greater than the thickness of the balance of thepipe, without the necessity for manufacturing the pipe in the firstinstance with zones of increased thickness, and without the necessity ofusing an external thickening collar. The hereindescribed procedure hasadditional advantages in that it:

Provides for molding detail such as trademarks, instructions, etc. intothe exterior of the bell.

Provides for a molded-in re-entrant pocket suitable for the reception ofa sealing O-ring.

Compensates for manufacturing variations in wall thickness.

Requires only inexpensive equipment so that tooling costs are low.

Makes possible the provision of a wide range of pipe sizes at practicalcost.

Makes possible molding chamfers in the bell margin.

Can be automated.

The manner in which the foregoing and other objects of this inventionare accomplished will be apparent from the accompanying specificationand claims considered together with the drawings wherein:

FIG. 1 is a view in side elevation of apparatus suitable for use inaccomplishing the hereindescribed method of shaping plastic pipe ortube.

FIGS. 2 and 3 are longitudinal, sectional views taken along line 2--2 ofFIG. 1, FIG. 2 illustrating the position of the apparatus at the startof the shaping operation and FIG. 3 illustrating the position of theapparatus at the conclusion thereof.

In its broad aspect, the hereindescribed method of shaping a normallyrigid plastic pipe or tube comprises first softening a selected zone ofthe pipe, usually its end. A resiliently deformable mandrel is insertedinto the softened zone. The mandrel is compressed axially andsimultaneously on its opposite sides by an amount predetermined toexpand it radially into frictional engagement with the softened pipezone.

While the mandrel is in such engagement, it is shortened by compressingit axially. This correspondingly expands the softened pipe radially andshortens it axially. Because of the resulting frictional drag, the sidewalls of the tube in the softened zone are thickened by an amountproportional to its shortening.

As additional features, there may be provided a rigid zone immediatelyadjacent the softened zone of the pipe and the mandrel may bedimensioned to engage both zones. Upon expansion of the mandrel, itreleasably locks itself to the rigid zone of the pipe. This creates apositive drive in the direction of shortening the softened zone andthickening its walls.

Also, an abutment may be provided against which the end face of thesoftened pipe zone bears as it is shortened. This further increases thepositive action of shortening the softened zone and thickening itswalls.

Still further, the softened zone of the pipe may be enclosed in a dieplaced in forming relation to it so that as it is expanded it is shapedto a desired contour, for example a socket or bell designed for use in apipe joint.

Considering the foregoing in greater detail and with particularreference to the drawings:

As stated above, the method of the invention is applicable particularlyto the shaping of pipe or tubing made of thermoplastic syntheticmaterials and used in such applications as the transmission of fluids,both pressured and unpressured, in encasing electrical duct work, etc.Such pipe is commonly manufactured by extruding softened plasticmaterial through a suitable die, cooling the resultant extruded pipe,and cutting it in lengths. The presently described apparatus may beemployed in conjunction with such a procedure, the ends of the pipelengths being heated until soft and then introduced into the apparatusfor shaping them into the desired bell or socket.

The apparatus employed for this purpose is indicated generally in FIG.1.

It comprises a base 10 having at its downstream end a substantialstandard 12. At its upstream end the base supports a standard 14 in theshape of an inverted U.

The latter standard mounts a separable die 16. The lower half 18 of thedie rests on base 10 between the two legs of standard 14. It mounts aplurality of upwardly extending guide rods 20.

The upper half 22 of the die is provided with openings which serve asslides and receive guide rods 20, thereby mounting the upper die halffor vertical reciprocation between open and closed positions.

The drive attached to the upper die part comprises a double acting,fluid operated cylinder 24. The base of the cylinder is supported on thecross piece of standard 14. The piston rod of the cylinder is integralwith the upper segment of the die. Reciprocation of the cylinderaccordingly reciprocates the upper segment of the die between open andclosed positions.

The interior of a die formed by mating segments 16, 22 is shaped to thedesired contour. As shown particularly in FIG. 2, the die cavity 26 maybe formed in the shape of a bell or socket of the desired diameter.Preferably it incorporates a re-entrant section 28 which in the moldedproduct forms an annular recess dimensioned to receive a sealing O-ring.

The die is open at the infeed end and is dimensioned to receive the endof a length of plastic pipe 30. This has been heated in a preliminarystep to form a softened zone to the right of dashed reference line 32FIG. 2, merging with a rigid zone to the left of the reference line.

Mandrel means are provided for insertion into the softened zone of thetube and expanding it to the dimensions of the die.

The mandrel basically comprises a hollow tube indicated generally at 34and made of a resiliently deformable material. Although a number of suchmaterials are suitable for the indicated purpose, a preferred onecomprises a natural or synthetic rubber, especially the synthetic rubbercommonly known as silicone rubber. Such rubbers have softening pointssufficiently elevated to remain unmelted at the softening temperature ofthe plastic tube i.e. at a temperature of about 400°F. Also, they may beprecisely shaped, are resiliently deformable, and when the deformingpressure has been removed return quickly and precisely to their originalcontour.

Mandrel 34 preferably is formed in an upstream segment 36 contoured as ahollow cylinder and a downstream segment 38 having an outward taper orflare. It is mounted on a segmented shaft or piston rod indicatedgenerally at 40.

At the outer end of the piston rod there is a reduced threaded segment42. Immediately adjacent is a segment 44 of normal diameter. At thejunction between these two segments there is a shoulder 46.

Next to segment 44 there is a segment 48 of reduced diameter. This formsa cavity 49 into which part of the substance of the mandrel is forcedduring its compression. It locates and determines a relatively thickzone in the pipe wall, as will appear hereinafter.

Immediately adjacent segment 48 of the piston rod there is a segment 50of normal diameter. This enters a cylinder 52 and is integral with astationary piston 54, housed in the cylinder. Fluid under pressure isintroduced alternately into cylinder 52 on opposite sides of piston 54through conduits 56, 58 in the usual manner.

Also integral with piston 54 is a piston rod segment 60. This terminatesin a threaded segment 62 of reduced diameter which forms with segment 60a shoulder 64. The threaded segment is received in a transverse openingin standard 12 and mounts a nut 66 by means of which the position of thepiston rod may be adjusted. Shoulder 64 bears against the inner face ofstandard 12 and, with nut 66 locks the piston rod, and piston, againstlongitudinal motion.

The piston rod thus is slidably received in a forward extension 68 ofcylinder 52 and a rearward extension 70 thereof. It mounts at its outerend an abutment or stop 72. This member of the assembly is in the fromof a ring having an outer diameter equal to or slightly less than theinner diameter of pipe 30. It is maintained releasably in position bymeans of a nut 74. Its inner face bears against shoulder 46 and alsoagainst the outer face of mandrel 34.

Cooperating with abutment 72 is an upstream abutment 76. This comprisesthe outer face of case extension 68. It bears against the inner end ofmandrel 34.

OPERATION

In the operation of the apparatus, the outer end of pipe 30 isheat-softened to a plastic condition. This is the end to the right ofreference line 32 of FIG. 2. The portion of the tube immediatelyadjacent to the left of the reference line is cool and still rigid.

With the die in its open position, the end of the pipe is insertedlongitudinally into the die opening until the end face of the pipe abutsagainst abutment surface 76. During this insertion, the softened end ofthe pipe is guided and flared outwardly into a contour corresponding tothe outwardly tapered or flared contour of the inner end 38 of mandrel34. This insures that the end of the pipe will be distended to the fullwidth of the final bell and not hang up by frictional engagement withabutment surface 76 during the forming operation.

The die then is closed. Fluid under pressure is admitted into theoperating cylinder through conduit 56 ahead of piston 54. Since thepiston is fixed, the result is to slide case 52 forwardly from its FIG.2 to its FIG. 3 position. This results in the compression of mandrel 34between abutment ring 72 and abutment surface 76.

During such compression, the mandrel expands radially while at the sametime shortening axially. Frictional engagement of the outer surface ofthe mandrel with the interior surface of the softened plastic pipecreates a frictional drag. This causes a thickening of the pipe wall atthe same time that the pipe is being distended radially and shortenedaxially until it conforms to the inner contour of the die.

This effect is augmented by reason of the fact that piston rod segment44 penetrates into the cool rigid zone of the tube, i.e. that to theleft of reference line 32. Expansion of the mandrel in this zonepositively but releasably locks the mandrel to the inner wall of therigid portion of the tube and creates a positive drive.

The thickening effect is augmented further by abutment of the inner faceof the tube against abutment surface 76.

The softened portion of the pipe thus is locked at both upstream anddownstream ends and is forced into a die cavity of known andpredetermined capacity. The result is that a bell or socket is formed onthe end of the pipe. After cooling of the bell to a rigidifyingtemperature, the piston is returned to its FIG. 2 position. This relaxesthe mandrel. The die then is opened and the pipe removed.

The resulting bell is of precise dimensions and reproducible from pipelength to pipe length even though there may be some irregularity in wallthickness of the pipe as originally extruded. The followingconsiderations prevail:

The initial forming conditions provide a mandrel volume V₁, and aremaining cavity volume V₂. When the plastic pipe with volume V₃ isintroduced into the cavity, the remaining unfilled volume will be (V₂ -V₃).

The distance that the cylinder 52 will move during the forming processis defined by the ratio of initial mandrel and plastic pipe volume tothe total volume of mandrel, plastic pipe, and cavity. It will moveuntil the cavity space is completely filled.

Thus if the plastic pipe is thin, then plastic pipe volume V₃ isreduced, and the remaining cavity volume (V₂ - V₃) is increased.Cylinder 52 must move farther to fill the cavity. This farther movementwill result in greater relative thickening of the pipe wall. Conversely,if the pipe is thick, then cylinder 52 does not move as far, so thatrelative thickening of the pipe is less.

The system thus compensates for variations in wall thickness to a degreedictated by the dimensions of the bell pocket, and the diameter of shaft40.

Further, because of the shortening of the pipe through the frictionaldrag effect, the wall thickness of the bell portion of the product mayassume any desired value, normally at least the thickness of the balanceof the pipe.

Still further, thicker sections of pipe wall can be produced in selectedpipe zones by the simple expedient of providing corresponding relievedsegments 48 in shaft 40. This is apparent from the followingconsiderations:

As cylinder 52 moves forward, cavity 26 will fill. The amount of motionof cylinder 52 required to fill the cavity will define the wallthickness of the pipe at this point in time. If shaft 40 is selectivelyreduced in diameter, as at 48, then further motion of cylinder 52 willbe required to fill the cavity. The mandrel and plastic pipe in the sameaxial zone will be shortened further, but the mandrel and plastic pipein the unrelieved zones at 44, 50 will not shorten. Thus a selectivelythicker section of pipe wall can be produced, in zones defined byrelieved areas 48.

Accordingly, by the practice of the invention, it is possible in asingle operation to provide on the end of a plastic pipe an integralbell or socket having a wall of any desired thickness and an annulargroove dimensioned to receive a sealing member such as an O-ring. Ofparticular significance is the fact that the thickened wall can becreated in any selected zone or area of the pipe.

This result is achieved, furthermore, without the necessity of usingcostly injection molds for the separate manufacture of bells of varioussizes, without the necessity of forming the pipe originally withthickened sections at spaced intervals, without the necessity of addingreinforcing rings to the bell area, and without delay of productionschedules.

We claim:
 1. Apparatus for shaping a normally rigid plastic tube havinga softened zone, comprising:a. a resiliently deformable mandreldimensioned for insertion into the softened zone and adjacent rigidportion of a tube to be shaped, b. mandrel pressing means engagingopposite ends of said mandrel for alternately pressing on the oppositeends of the mandrel for simultaneously expanding it radially andshortening it axially, and thereafter relaxing it to permit it to returnto its original dimension, and c. die means surrounding the mandrel andpositioned for contact by a rigid portion of a tube to be shapedadjacent the softened zone thereof and for forming contact by thesoftened zone of the tube during compression of the mandrel.
 2. Theapparatus of claim 1 wherein the mandrel pressing means includes movableabutment means arranged for abutting the adjacent end face of a tube tobe shaped.
 3. The apparatus of claim 1 whereina. the mandrel comprises ahollow cylinder made of a resiliently deformable material anddimensioned for insertion inside a plastic tube to be shaped, and b. themandrel pressing means comprises1. a piston rod extending within the diemeans and fixed at its outer end relative to the die means and mountingthe mandrel adjacent its inner end,
 2. a first abutment rigidly mountedon the inner end of the piston rod and arranged for bearing engagementwith the adjacent end of the mandrel,
 3. a second abutment freelyslidable on the piston rod and extending into the die for engagementwith both the inner end of the mandrel and the adjacent face of the endof a tube to be shaped,
 4. a fluid pressure cylinder integral with andextending outwardly from the second abutment means and freely encirclingthe piston rod, and
 5. a piston fixed to the piston rod within thecylinder, whereby the application of fluid pressure selectively toopposite ends of the cylinder effects selective extension and retractionof the second abutment relative to the die means and said alternatepressing and relaxing of the mandrel.
 4. Apparatus for shaping anormally rigid plastic tube having a softened end zone, the apparatuscomprisinga. a mandrel comprising a hollow cylinder made of aresiliently deformable rubber and dimensioned for insertion inside thesoftened end zone and adjacent rigid portions of a plastic tube to beshaped, b. a piston rod having inner and outer ends and mounting themandrel adjacent said inner end, c. die means freely surrounding themandrel and arranged to surround the softened end zone and an adjacentrigid portion of a tube to be shaped, d. a first abutment rigidlymounted on the inner end of a piston rod and arranged for bearingengagement with the adjacent end of the mandrel, e. a second abutmentfreely slidable on the piston rod and extending into the die means forbearing engagement with both the adjacent end of the mandrel and theface of the softened end of a tube to be shaped, f. a fluid pressurecylinder integral with and extending outwardly from the second abutmentand freely encircling the piston rod intermediate the ends of thelatter, and g. a piston fixed to the piston rod within the cylinder,whereby the application of fluid pressure to one end of the cylindereffects movement of the cylinder along the piston rod and extension ofthe second abutment into the die means, thereby compressing the mandrelbetween the first and second abutments and expanding the mandrelradially while simultaneously shortening it axially, contemporaneouslysecuring the plastic tube and mandrel within the die means against axialdisplacement and expanding the softened end zone of the plastic tuberadially outward into forming contact with the die means, and theapplication of fluid pressure to the opposite end of the cylindereffects retraction of the cylinder and second abutment outwardlyrelative to the die means to return the mandrel to its original relaxedcondition.
 5. The apparatus of claim 4 wherein the piston rod comprisesa segmented shaft having in a predetermined location opposite a selectedarea of the die means a relieved segment of reduced diameter which inthe uncompressed condition of the mandrel is spaced therefrom, therebyforming a cavity positioned for receiving a portion of the mandrel uponcompression of the latter.
 6. Apparatus for shaping a normally rigidplastic tube having a softened zone, comprising:a. a resilientlydeformable mandrel dimensioned for insertion into the softened zone andadjacent rigid portion of a tube to be shaped, the inner end of themandrel being flared outwardly, b. mandrel pressing means engagingopposite ends of said mandrel for alternately pressing on the oppositeends of the mandrel for simultaneously expanding it radially andshortening it axially, and thereafter relaxing it to permit it to returnto its original dimension, the mandrel pressing means including movableabutment means arranged for abutting the end face of the softened endzone of a tube to be shaped, and c. die means surrounding the mandreland positioned for contact by a rigid portion of a tube to be shapedadjacent the softened zone thereof and for forming contact by thesoftened zone of the tube during compression of the mandrel.